The present invention relates to organic electroluminescent devices having at least two emitting layers, where the two emitting layers comprise phosphorescent dopants.
The structure of organic electroluminescent devices (OLEDs) in which organic semiconductors are employed as functional materials is described, for example, in U.S. Pat. Nos. 4,539,507, 5,151,629, EP 0676461 and WO 98/ 27136. A development in the area of organic electroluminescent devices are white-emitting OLEDs. These can be employed either for monochrome white displays or, with coloured filters, for full-colour displays. They are furthermore suitable for lighting applications. White-emitting organic electroluminescent devices based on low-molecular-weight compounds generally have at least two emission layers. Electroluminescent devices having precisely two emission layers are used, in particular, for passive matrix applications, where in accordance with the prior art frequently only fluorescent emitters are used for this purpose. The electroluminescent devices usually have at least three emission layers, which exhibit blue, green and orange or red emission. Either fluorescent or phosphorescent emitters are used in the emission layers, where the phosphorescent emitters exhibit significant advantages owing to the higher achievable efficiency. The general structure of a white-emitting OLED of this type having at least one phosphorescent layer is described, for example, in WO 2005/011013. Owing to the higher achievable efficiency, a white-emitting OLED which comprises only phosphorescent emitter layers would be desirable.
However, since blue-phosphorescent emitters generally do not yet satisfy the common requirements, in particular with respect to the operating lifetime, hybrid OLEDs, i.e. a fluorescent blue emitter layer combined with phosphorescent orange or red and green emitter layers (in the case of three-colour white) or a fluorescent blue emitter layer combined with a phosphorescent yellow to orange emitter layer (in the case of two-colour white), are used in most applications in accordance with the prior art.
A fundamental problem of such hybrid OLEDs consists in that common matrix and emitter materials used in the blue-fluorescent emitter layer generally have an excessively low triplet level for phosphorescent dopants, which can result in triplet excitons being extinguished via the blue emitter layer. This results in lower efficiency of the OLED. In order to obtain maximum efficiency from a white hybrid OLED, care must be taken to prevent this extinguishing of triplet excitons. This is possible through the use of an organic interlayer between the phosphorescent emitter layer and the fluorescent emitter layer. However, very high requirements are made of the materials of an interlayer of this type in order that they on the one hand prevent the extinguishing of triplet excitons, but on the other hand also have no adverse effects on efficiency, lifetime or voltage of the organic electroluminescent device, meaning that it can be difficult to achieve an interlayer of this type.
A further possibility in accordance with the prior art of preventing the extinguishing of triplet excitons via the fluorescent layer consists in implementing the OLED as so-called “stacked OLED” or “tandem OLED”, where the phosphorescent layer or layers are arranged in one of the electroluminescent units and the fluorescent layer or layers are arranged in another of the electroluminescent units (for example Y.-S. Tyan et al., SID-Symposium Digest, 2009, 895). Direct contact between the fluorescent emitter layer and the phosphorescent emitter layer can thus be prevented. This structure has the advantage that an organic interlayer is not necessary between the fluorescent emitter layer and the phosphorescent emitter layer. Furthermore, each individual electroluminescent unit of the tandem OLED is subjected to a lower current load than would be the case if all emitting layers were arranged directly one on top of another.
In a tandem OLED, two or more electroluminescent units are connected vertically in series, where charge-generation layers are present between the individual electroluminescent units (for example T.-W. Lee et al., Appl. Phys. Lett. 2008, 92, 043301). The charge-generation layer is usually formed by coupling an n-conducting layer (or a conducting electron-injection layer) and a p-conducting layer (or a conducting hole-injection layer) to one another. The p-conducting layers used in accordance with the prior art comprise, for example, p-doped organic hole-transport materials, where the dopant is, for example, F4-TCNQ or WO3, or inorganic materials, such as, for example, indium tin oxide (ITO), V2O5, WO3 or MoO3. The n-conducting layer is generally a doped organic electron-transport layer, where the dopant used comprises metals having a low work function, such as, for example, Cs, Li or Mg, or metal carbonates.
In general, it is not sufficient to use only a yellow-phosphorescent dopant in the phosphorescent emitter layer. Thus, a green-phosphorescent dopant and an orange- or red-phosphorescent dopant are usually combined in the phosphorescent electroluminescent unit of the tandem OLED. This can be carried out either by doping both dopants into the same emitter layer or by the two dopants being present in separate emitter layers. However, there are disadvantages in both methods. Thus, a comparatively poor lifetime is obtained if both phosphorescent emitters are doped into one emitter layer. By contrast, although a good lifetime is obtained if the two phosphorescent emitters are present in two separate emitter layers, a strong colour shift with the lifetime which cannot be attributed to the ageing of one of the emitters is, however, observed.
Since it is generally not an advance in practice if only the operating lifetime is improved, but this is accompanied by a relatively large colour shift, the technical problem on which the present invention is based is therefore to provide a device architecture for these two phosphorescent layers which has a comparatively good or improved lifetime of the phosphorescent emission unit and at the same time a small colour shift with the lifetime.
Surprisingly, it has been found that an OLED having the structure defined below solves this problem and results in a very small colour shift at the same time as a very good lifetime.